Driver

ABSTRACT

A driver or dogging element adapted to transfer torsion forces between a rotating outer tube ( 10 ) and a drill bit ( 1 ) of a down-the-hole hammer, wherein the dogging element ( 7 ) is made of a softer material than those parts ( 3, 4 ) of the drill bit ( 1 ) that co-act with the dogging element ( 7, 8 ).

The present invention relates to a driver or dogging element fortransferring torsion forces between a rotating outer tube and the drillbit of a down-the-hole hammer.

In the case of countersink hammers, rotational forces, feed, andpercussion energy are transferred from a drilling unit to adown-the-hole hammer that is lowered into the hole to be drilled. Therotational forces are transferred with the aid of a rotating outer tubesuch as to cause the drill bit to rotate, and the percussion forces aretransferred with the aid of a piston or ram that moves reciprocatinglyin the tube in its longitudinal direction. The rotational force istransferred from the outer tube by means of a chuck or dogging elementscrewed firmly into the outer tube. Arranged inside the dogging elementor chuck is an array of splines which co-act with splines arranged onthe outside of the drill bit shaft. The drill bit, the outer tube andthe dogging element or chuck are all made of steel.

This technique has been used with satisfaction for many years. Becauseof the greater powers to which a drilling unit is subjected, resultingin higher drilling speeds, the stresses and strains to which componentmaterials are subjected have also increased. In the present case, thishas resulted in greater wear on the spline transmission which, in turn,has led to the formation of cracks and fissures which, in turn, have ledto breakdowns caused by fracturing of the drill bit shaft. This canresult in the loss of the drill bit, with the drill bit remaining in thedrill hole. It may lead to that the lost drill bit cannot be recoveredand that a new hole must be drilled from the beginning.

Accordingly, an object of the present invention is to provide for thetransfer of torsion forces between a rotating outer tube and a drill bitof a down-the-hole hammer a novel driver or dogging element thatovercomes the aforesaid problems.

This object is achieved with a driver or dogging element according tothe present invention. The invention is characterised in that thedogging element is made from a material that is softer than the materialfrom which those parts of the drill bit that co-act with the doggingelement are made.

The invention will now be described in more detail with reference to anexemplifying embodiment and also with reference to the accompanyingdrawings, in which

FIG. 1 is a longitudinally sectioned view of a drill bit for adown-the-hole hammer and shows those parts that are in closestconnection with the drill bit; and

FIG. 2 is a cross-sectional view of an inventive dogging elementincluded in the FIG. 1 embodiment.

FIG. 1 thus shows a drill bit 1 that has rock drilling buttons 2 mountedon its front end in a known manner. Typically, the drill bit 1 alsoincludes a shaft 3 which is provided with longitudinally extendingsplines 4 along a part of its length. The drill bit also has a rear end5 against which a percussion piston is intended to strike. The drill bit1 typically includes a longitudinally extending channel 6 through whichcompressed air is blown out from the front part of the drill to blow thedrill cuttings away from the drill bit and out of the drill hole.

The shaft 3 carries a driver or dogging element 7 which is providedinternally with longitudinally extending splines 8 that co-act with thesplines 4. The dogging element 7 is generally in the form of a sleeveand is provided along at least part of its length with an external screwthread 9 by means of which the dogging element 7 can be screwed securelyto the front end of an outer tube 10 that surrounds at least a part ofboth the dogging element 7 and the shaft 3 of the drill bit 1. The outertube 10 also surrounds the percussion piston 11. The down-the-holehammer can be spliced or joined to allow the drill run to be extended toa desired length. The drilling unit causes the outer tube 10 to rotate,wherewith the rotational movement of the tube is transmitted to thedogging element 7 which, in turn, transfers the rotational movement tothe drill bit 1 through the medium of the spline connection 8, 4.

According to the invention, the dogging element 7 is made of a materialthat is significantly softer than those parts of the drill bit 1 thatco-act with the dogging element, i.e. than the shaft 3, therewithavoiding those problems associated with the formation of cracks,fissures, and with breakage of the shaft 3. It has been found that abronze alloy is a suitable material for the dogging element. This alloymay be a tin bronze alloy or a chromium bronze alloy. A tin bronze alloyproduced by Applicants Atlas Copco Secoroc AB and designated 11 9351 93has been found to be excellent in this respect.

When the dogging element 7 is made of this softer material, it is thismaterial that suffers the wear and tear rather than the steel in theshaft 3 of the drill bit co-acting with the dogging element. As thedogging element 7 rotates in response to the rotation of the outer tube10, force is transmitted by the splines 8 on the dogging element to thesplines 4 on the shaft 3, wherewith the splines 8 on the doggingelement, which are softer than the splines 4, will become worn insteadof the splines 4 on shaft of the drill bit 1. There is thus obtained acontrolled and observable wear and the dogging element can be replacedwhen it is noticed that the degree of wear on the dogging element makesthis necessary. The splines 8 on the dogging element may be allowed towear down to about half their original width before needing to replacethe dogging element. Breakage or fracture of the dogging element 7 willnot cause any serious damage, since it is only the dogging element thatbreaks while the drill bit will still be held firmly by the outer tube10.

The preferred material is not only softer than steel, but also hashigher thermal conductivity and lower friction than steel, meaning thatthe stresses and strains in the spline connection will be much lowerthan those experienced with known techniques using steel on steelcontact.

1. A dogging element adapted to transfer torsion forces between arotating outer tube (10) and a drill bit (1) of a down-the-hole hammer,characterised in that the drill bit (1) comprises a shaft (3) made ofsteel and that the dogging element (7) is made of a softer material thanthe parts (3, 4) of the drill bit (1) that co-act with the doggingelement (7, 8).
 2. A dogging element according to claim 1, characterisedin that the dogging element (7) is provided internally with splines (8)which are intended for co-action with splines (4) provided on the shaft(3) of the drill bit (1).
 3. A dogging element according to claim 1,characterised in that the dogging element (7) is made of a bronze alloy.4. A dogging element according to claim 3, characterised in that thedogging element (7) is made of tin bronze material.
 5. A dogging elementaccording to claim 3, characterised in that the dogging element (7) ismade of a chromium bronze material.
 6. A dogging element according toclaim 1, characterised in that the dogging element (7) is made of amterial that has high thermal conductivity.